1. Field of the Invention
This invention relates to cordless telephones, and more particularly to a cordless telephone which includes an alternative configuration for use without power to a base unit.
2. Background of Related Art
Cordless remote telephones are popular consumer devices which allow a user in a home or office the freedom to stray hundreds or even thousands of feet from a base unit. Initially, the handsets of cordless telephones communicated with their base unit using analog signals. In more recent years, advancements have been made with respect to cordless telephones allowing digital communications between the handset and its base unit.
With the advancement in technology of cordless telephones, processors such as digital signal processors or microprocessors have been included within the base unit and/or the handset of a cordless telephone. The processors provide the cordless telephone with a vast array of features, but require a power source to operate.
Since the early days of telephones, a small amount of power has typically been provided on a telephone line by a central office for use by a telephone (customer premises equipment) connected to the telephone line. The power was typically used by the central office to sense on- and off-hook conditions, and by the user as a byproduct, e.g., to illuminate the telephone dial or keypad, and/or to bias a microphonic device, and/or to power a dual tone, multiple frequency (DTMF) device at the customer premises equipment. Even though the power provided by the telephone central office may be sufficient to power some conventional telephone devices such as standard wired handsets, there is nevertheless generally insufficient current available within regulatory guidelines to power the components of a cordless telephone. Thus, cordless telephones have generally and conventionally required external power sources.
The power source for a base unit of a cordless telephone typically is general household current, e.g., 120 volts AC, 50-60 Hz in the United States. The handset of the cordless telephone is typically powered by a battery. In most cases, the externally powered base unit also provides charging power for the battery in the handset when coupled with the base unit.
For all the advantages provided by the use of processors, RF transceivers and/or other externally powered devices, a conventional cordless telephone is nevertheless rendered useless during a power failure.
For instance, power may be removed from the base unit when severe weather conditions knock power lines to the home or office out of service. In such conditions, it may be desirable to utilize the telephone, particularly in emergency or life threatening situations. Without power, the conventional cordless telephone is incapable of placing or receiving telephone calls.
FIG. 4 depicts a conventional cordless telephone including a base unit 400 and a handset 450. The base unit 400 and the handset 450 each include a respective antenna 414, 466 for use in transmitting a radio frequency (RF) signal therebetween.
The base unit 400 includes a telephone line interface (TLI) 410 and cable 412 (e.g., including an RJ-11 telephone jack) for interconnection with a central office via a standard wall jack or the like. Although available, the conventional base unit typically does not draw a significant amount of power from the telephone line for powering devices therein. Of course, power is drawn from the telephone line to provide proper DC and AC characteristics to the central office. However, an external AC power source is utilized to power the components of the conventional cordless telephone.
A standard power cable 402 provides standard AC power to an alternating current (AC)-to-direct current (DC) converter 404. AC/DC converter 404 converts power from the external power source into DC power for use by a processor 408, RF transceiver 406, and other externally powered devices within the base unit 400. If the telephone line interface (TLI) 410 (otherwise known as a hybrid) is an active device, then it too is powered by the AC-to-DC converter 404. The AC/DC converter 404 is typically protected by a diode 407 or other isolating device.
A conventional handset 450 typically includes, besides antenna 466, an RF transceiver 454, a processor 456 such as a DSP, microcontroller or microprocessor, an analog-to-digital (AND) converter 458, and a digital-to-analog (D/A) converter 460, all powered by a battery pack 452. A microphone 462 provides an analog signal to the A/D converter 458, and a speaker 464 outputs an analog signal from D/A converter 460. A conventional 12 or 16-key keypad 467 allows direct user input and standard operation of the cordless telephone
During normal operation, the base unit 400 communicates with its handset 450 using a processed RF signal transmitted between antennae 414, 466. Unfortunately, as discussed above, a power failure will render the conventional base unit 400 virtually useless because the RF transceiver 406 and the DSP 408 will not be operational for lack of power. This is not a desirable situation, and in emergency situations may be life threatening if a separate, non-cordless telephone is not available. This causes the need to maintain an entirely separate corded telephone in another location in the home or office, whether or not a cordless telephone is also in service.
There is a need to provide a cordless telephone which has the ability, particularly in emergency situations, to be operated even when power is removed from the base unit.